1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device. Embodiments of the invention have particular relevance to a method and apparatus for adjusting the settings of the lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Where a manufacturing facility has more than one lithographic apparatus used for printing the same product types, it is desirable to be able to match the characteristics of all the lithographic apparatus to each other. This will allow the apparatus to be used in a “mix and match” configuration, in which the apparatus may be arranged to have the same optical transfer function.
In one lithographic manufacturing process, a first lithographic transfer function of a first lithographic projection apparatus is obtained. The information is compared with information relating to a second lithographic transfer function corresponding to a second lithographic projection apparatus, which acts as a reference apparatus. The lithographic transfer function describes the transfer of spatial frequencies from the pattern on the mask to the pattern projected on the substrate. The difference between the first and second information is calculated. Machine settings for the first lithographic apparatus are changed in order to minimize the difference so as to improve the match between the first and second lithographic projection apparatus. The information that is obtained on each of the first and second projection apparatus is critical dimension (CD) pitch anomaly. Variations in CD pitch can occur as a function of variations in the machine settings of a lithography apparatus, such as exposure energy and illumination settings.
However, there is a problem with this method of matching two lithographic projection apparatus in that the measurement of CD does not allow discrimination of the various lithographic projection apparatus parameters, thus leading to errors.